The overall goal of this study is to understand the mechanism underlying transcription factors specifying osteoclast lineage commitment and differentiation. This proposal is highly significant since elucidating osteoclast lineage commitment and differentiation has potential to define new therapeutic targets for bone disorders that involve osteoclast generation and activation. Despite the recent insights gained from the effects of targeted deletion of the c-fos, PU.1, NF-[unreadable]B, and NFATc1 transcription factor genes, the mechanism underlying transcription factors specifying osteoclast (OC) lino define new therapeutic targets for bone disorders that involve osteoclast generation and activation. Despite the recent insights gained from the effects of targeted deletion of the c-fos, PU.1, NF-[unreadable]monstrated that AML1 is highly induced by RANKL and M-CSF together. AML1 knockdown in mouse bone marrow culture induced by RANKL and M-CSF blocked osteoclast differentiation, but did not inhibit macrophage differentiation. However, AML1-/- liver cells failed to develop both monocytes/macrophages and osteoclasts. Our results showed that that AML1 may control osteoclast cell lineage commitment and regulate osteoclast gene expression and differentiation through upregulating PU.1 and NFATc1. Based on our Preliminary study, we hypothesize that AML1 is a key regulator that specifies osteoclast cell lineage commitment and differentiation at the transcriptional regulation level. We will test this hypothesis through two specific aims. We will define the functional role of AML1 in osteoclast cell lineage commitment and differentiation using RNAi knockdown and overexpression in Aim 1. We will investigate the role of AML1 in osteoclast differentiation in adult mice through bone tissue-specific targeted disruption of AML using a conditional knockout approach by Cre/loxP technology and characterize the phenotypes and pathomechanism of the AML1 conditional knockout mice. Ultimately, this knowledge will help to establish the roles of AML1 in osteoclast cell lineage commitment and differentiation. Thus, it will improve our understanding of osteolytic diseases and help to design novel approaches for the treatment of diseases such as osteoporosis, arthritis, periodontal disease, and bone metastases using drug or somatic gene therapy.